1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0

// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.

//! Implementations of the `IntoConnectionHandler` and `ConnectionHandler` traits for both incoming
//! and outgoing substreams for all gossiping protocols.
//!
//! This is the main implementation of `ConnectionHandler` in this crate, that handles all the
//! gossiping protocols that are Substrate-related and outside of the scope of libp2p.
//!
//! # Usage
//!
//! From an API perspective, for each of its protocols, the [`NotifsHandler`] is always in one of
//! the following state (see [`State`]):
//!
//! - Closed substream. This is the initial state.
//! - Closed substream, but remote desires them to be open.
//! - Open substream.
//! - Open substream, but remote desires them to be closed.
//!
//! Each protocol in the [`NotifsHandler`] can spontaneously switch between these states:
//!
//! - "Closed substream" to "Closed substream but open desired". When that happens, a
//! [`NotifsHandlerOut::OpenDesiredByRemote`] is emitted.
//! - "Closed substream but open desired" to "Closed substream" (i.e. the remote has cancelled
//! their request). When that happens, a [`NotifsHandlerOut::CloseDesired`] is emitted.
//! - "Open substream" to "Open substream but close desired". When that happens, a
//! [`NotifsHandlerOut::CloseDesired`] is emitted.
//!
//! The user can instruct a protocol in the `NotifsHandler` to switch from "closed" to "open" or
//! vice-versa by sending either a [`NotifsHandlerIn::Open`] or a [`NotifsHandlerIn::Close`]. The
//! `NotifsHandler` must answer with [`NotifsHandlerOut::OpenResultOk`] or
//! [`NotifsHandlerOut::OpenResultErr`], or with [`NotifsHandlerOut::CloseResult`].
//!
//! When a [`NotifsHandlerOut::OpenResultOk`] is emitted, the substream is now in the open state.
//! When a [`NotifsHandlerOut::OpenResultErr`] or [`NotifsHandlerOut::CloseResult`] is emitted,
//! the `NotifsHandler` is now (or remains) in the closed state.
//!
//! When a [`NotifsHandlerOut::OpenDesiredByRemote`] is emitted, the user should always send back
//! either a [`NotifsHandlerIn::Open`] or a [`NotifsHandlerIn::Close`].If this isn't done, the
//! remote will be left in a pending state.
//!
//! It is illegal to send a [`NotifsHandlerIn::Open`] before a previously-emitted
//! [`NotifsHandlerIn::Open`] has gotten an answer.

use crate::{
	protocol::notifications::upgrade::{
		NotificationsIn, NotificationsInSubstream, NotificationsOut, NotificationsOutSubstream,
		UpgradeCollec,
	},
	types::ProtocolName,
};

use bytes::BytesMut;
use futures::{
	channel::mpsc,
	lock::{Mutex as FuturesMutex, MutexGuard as FuturesMutexGuard},
	prelude::*,
};
use libp2p::{
	core::ConnectedPoint,
	swarm::{
		handler::ConnectionEvent, ConnectionHandler, ConnectionHandlerEvent, KeepAlive,
		NegotiatedSubstream, SubstreamProtocol,
	},
	PeerId,
};
use log::error;
use parking_lot::{Mutex, RwLock};
use std::{
	collections::VecDeque,
	mem,
	pin::Pin,
	sync::Arc,
	task::{Context, Poll},
	time::{Duration, Instant},
};

/// Number of pending notifications in asynchronous contexts.
/// See [`NotificationsSink::reserve_notification`] for context.
const ASYNC_NOTIFICATIONS_BUFFER_SIZE: usize = 8;

/// Number of pending notifications in synchronous contexts.
const SYNC_NOTIFICATIONS_BUFFER_SIZE: usize = 2048;

/// Maximum duration to open a substream and receive the handshake message. After that, we
/// consider that we failed to open the substream.
const OPEN_TIMEOUT: Duration = Duration::from_secs(10);

/// After successfully establishing a connection with the remote, we keep the connection open for
/// at least this amount of time in order to give the rest of the code the chance to notify us to
/// open substreams.
const INITIAL_KEEPALIVE_TIME: Duration = Duration::from_secs(5);

/// The actual handler once the connection has been established.
///
/// See the documentation at the module level for more information.
pub struct NotifsHandler {
	/// List of notification protocols, specified by the user at initialization.
	protocols: Vec<Protocol>,

	/// When the connection with the remote has been successfully established.
	when_connection_open: Instant,

	/// Whether we are the connection dialer or listener.
	endpoint: ConnectedPoint,

	/// Remote we are connected to.
	peer_id: PeerId,

	/// Events to return in priority from `poll`.
	events_queue: VecDeque<
		ConnectionHandlerEvent<NotificationsOut, usize, NotifsHandlerOut, NotifsHandlerError>,
	>,
}

impl NotifsHandler {
	/// Creates new [`NotifsHandler`].
	pub fn new(peer_id: PeerId, endpoint: ConnectedPoint, protocols: Vec<ProtocolConfig>) -> Self {
		Self {
			protocols: protocols
				.into_iter()
				.map(|config| {
					let in_upgrade = NotificationsIn::new(
						config.name.clone(),
						config.fallback_names.clone(),
						config.max_notification_size,
					);

					Protocol { config, in_upgrade, state: State::Closed { pending_opening: false } }
				})
				.collect(),
			peer_id,
			endpoint,
			when_connection_open: Instant::now(),
			events_queue: VecDeque::with_capacity(16),
		}
	}
}

/// Configuration for a notifications protocol.
#[derive(Debug, Clone)]
pub struct ProtocolConfig {
	/// Name of the protocol.
	pub name: ProtocolName,
	/// Names of the protocol to use if the main one isn't available.
	pub fallback_names: Vec<ProtocolName>,
	/// Handshake of the protocol. The `RwLock` is locked every time a new substream is opened.
	pub handshake: Arc<RwLock<Vec<u8>>>,
	/// Maximum allowed size for a notification.
	pub max_notification_size: u64,
}

/// Fields specific for each individual protocol.
struct Protocol {
	/// Other fields.
	config: ProtocolConfig,

	/// Prototype for the inbound upgrade.
	in_upgrade: NotificationsIn,

	/// Current state of the substreams for this protocol.
	state: State,
}

/// See the module-level documentation to learn about the meaning of these variants.
enum State {
	/// Protocol is in the "Closed" state.
	Closed {
		/// True if an outgoing substream is still in the process of being opened.
		pending_opening: bool,
	},

	/// Protocol is in the "Closed" state. A [`NotifsHandlerOut::OpenDesiredByRemote`] has been
	/// emitted.
	OpenDesiredByRemote {
		/// Substream opened by the remote and that hasn't been accepted/rejected yet.
		in_substream: NotificationsInSubstream<NegotiatedSubstream>,

		/// See [`State::Closed::pending_opening`].
		pending_opening: bool,
	},

	/// Protocol is in the "Closed" state, but has received a [`NotifsHandlerIn::Open`] and is
	/// consequently trying to open the various notifications substreams.
	///
	/// A [`NotifsHandlerOut::OpenResultOk`] or a [`NotifsHandlerOut::OpenResultErr`] event must
	/// be emitted when transitionning to respectively [`State::Open`] or [`State::Closed`].
	Opening {
		/// Substream opened by the remote. If `Some`, has been accepted.
		in_substream: Option<NotificationsInSubstream<NegotiatedSubstream>>,
		/// Is the connection inbound.
		inbound: bool,
	},

	/// Protocol is in the "Open" state.
	Open {
		/// Contains the two `Receiver`s connected to the [`NotificationsSink`] that has been
		/// sent out. The notifications to send out can be pulled from this receivers.
		/// We use two different channels in order to have two different channel sizes, but from
		/// the receiving point of view, the two channels are the same.
		/// The receivers are fused in case the user drops the [`NotificationsSink`] entirely.
		notifications_sink_rx: stream::Peekable<
			stream::Select<
				stream::Fuse<mpsc::Receiver<NotificationsSinkMessage>>,
				stream::Fuse<mpsc::Receiver<NotificationsSinkMessage>>,
			>,
		>,

		/// Outbound substream that has been accepted by the remote.
		///
		/// Always `Some` on transition to [`State::Open`]. Switched to `None` only if the remote
		/// closed the substream. If `None`, a [`NotifsHandlerOut::CloseDesired`] event has been
		/// emitted.
		out_substream: Option<NotificationsOutSubstream<NegotiatedSubstream>>,

		/// Substream opened by the remote.
		///
		/// Contrary to the `out_substream` field, operations continue as normal even if the
		/// substream has been closed by the remote. A `None` is treated the same way as if there
		/// was an idle substream.
		in_substream: Option<NotificationsInSubstream<NegotiatedSubstream>>,
	},
}

/// Event that can be received by a `NotifsHandler`.
#[derive(Debug, Clone)]
pub enum NotifsHandlerIn {
	/// Instruct the handler to open the notification substreams.
	///
	/// Must always be answered by a [`NotifsHandlerOut::OpenResultOk`] or a
	/// [`NotifsHandlerOut::OpenResultErr`] event.
	///
	/// Importantly, it is forbidden to send a [`NotifsHandlerIn::Open`] while a previous one is
	/// already in the fly. It is however possible if a `Close` is still in the fly.
	Open {
		/// Index of the protocol in the list of protocols passed at initialization.
		protocol_index: usize,
	},

	/// Instruct the handler to close the notification substreams, or reject any pending incoming
	/// substream request.
	///
	/// Must always be answered by a [`NotifsHandlerOut::CloseResult`] event.
	Close {
		/// Index of the protocol in the list of protocols passed at initialization.
		protocol_index: usize,
	},
}

/// Event that can be emitted by a `NotifsHandler`.
#[derive(Debug)]
pub enum NotifsHandlerOut {
	/// Acknowledges a [`NotifsHandlerIn::Open`].
	OpenResultOk {
		/// Index of the protocol in the list of protocols passed at initialization.
		protocol_index: usize,
		/// Name of the protocol that was actually negotiated, if the default one wasn't available.
		negotiated_fallback: Option<ProtocolName>,
		/// The endpoint of the connection that is open for custom protocols.
		endpoint: ConnectedPoint,
		/// Handshake that was sent to us.
		/// This is normally a "Status" message, but this out of the concern of this code.
		received_handshake: Vec<u8>,
		/// How notifications can be sent to this node.
		notifications_sink: NotificationsSink,
		/// Is the connection inbound.
		inbound: bool,
	},

	/// Acknowledges a [`NotifsHandlerIn::Open`]. The remote has refused the attempt to open
	/// notification substreams.
	OpenResultErr {
		/// Index of the protocol in the list of protocols passed at initialization.
		protocol_index: usize,
	},

	/// Acknowledges a [`NotifsHandlerIn::Close`].
	CloseResult {
		/// Index of the protocol in the list of protocols passed at initialization.
		protocol_index: usize,
	},

	/// The remote would like the substreams to be open. Send a [`NotifsHandlerIn::Open`] or a
	/// [`NotifsHandlerIn::Close`] in order to either accept or deny this request. If a
	/// [`NotifsHandlerIn::Open`] or [`NotifsHandlerIn::Close`] has been sent before and has not
	/// yet been acknowledged by a matching [`NotifsHandlerOut`], then you don't need to a send
	/// another [`NotifsHandlerIn`].
	OpenDesiredByRemote {
		/// Index of the protocol in the list of protocols passed at initialization.
		protocol_index: usize,
	},

	/// The remote would like the substreams to be closed. Send a [`NotifsHandlerIn::Close`] in
	/// order to close them. If a [`NotifsHandlerIn::Close`] has been sent before and has not yet
	/// been acknowledged by a [`NotifsHandlerOut::CloseResult`], then you don't need to a send
	/// another one.
	CloseDesired {
		/// Index of the protocol in the list of protocols passed at initialization.
		protocol_index: usize,
	},

	/// Received a message on a custom protocol substream.
	///
	/// Can only happen when the handler is in the open state.
	Notification {
		/// Index of the protocol in the list of protocols passed at initialization.
		protocol_index: usize,
		/// Message that has been received.
		message: BytesMut,
	},
}

/// Sink connected directly to the node background task. Allows sending notifications to the peer.
///
/// Can be cloned in order to obtain multiple references to the substream of the same peer.
#[derive(Debug, Clone)]
pub struct NotificationsSink {
	inner: Arc<NotificationsSinkInner>,
}

#[derive(Debug)]
struct NotificationsSinkInner {
	/// Target of the sink.
	peer_id: PeerId,
	/// Sender to use in asynchronous contexts. Uses an asynchronous mutex.
	async_channel: FuturesMutex<mpsc::Sender<NotificationsSinkMessage>>,
	/// Sender to use in synchronous contexts. Uses a synchronous mutex.
	/// Contains `None` if the channel was full at some point, in which case the channel will
	/// be closed in the near future anyway.
	/// This channel has a large capacity and is meant to be used in contexts where
	/// back-pressure cannot be properly exerted.
	/// It will be removed in a future version.
	sync_channel: Mutex<Option<mpsc::Sender<NotificationsSinkMessage>>>,
}

/// Message emitted through the [`NotificationsSink`] and processed by the background task
/// dedicated to the peer.
#[derive(Debug)]
enum NotificationsSinkMessage {
	/// Message emitted by [`NotificationsSink::reserve_notification`] and
	/// [`NotificationsSink::write_notification_now`].
	Notification { message: Vec<u8> },

	/// Must close the connection.
	ForceClose,
}

impl NotificationsSink {
	/// Returns the [`PeerId`] the sink is connected to.
	pub fn peer_id(&self) -> &PeerId {
		&self.inner.peer_id
	}

	/// Sends a notification to the peer.
	///
	/// If too many messages are already buffered, the notification is silently discarded and the
	/// connection to the peer will be closed shortly after.
	///
	/// The protocol name is expected to be checked ahead of calling this method. It is a logic
	/// error to send a notification using an unknown protocol.
	///
	/// This method will be removed in a future version.
	pub fn send_sync_notification(&self, message: impl Into<Vec<u8>>) {
		let mut lock = self.inner.sync_channel.lock();

		if let Some(tx) = lock.as_mut() {
			let result =
				tx.try_send(NotificationsSinkMessage::Notification { message: message.into() });

			if result.is_err() {
				// Cloning the `mpsc::Sender` guarantees the allocation of an extra spot in the
				// buffer, and therefore `try_send` will succeed.
				let _result2 = tx.clone().try_send(NotificationsSinkMessage::ForceClose);
				debug_assert!(_result2.map(|()| true).unwrap_or_else(|err| err.is_disconnected()));

				// Destroy the sender in order to not send more `ForceClose` messages.
				*lock = None;
			}
		}
	}

	/// Wait until the remote is ready to accept a notification.
	///
	/// Returns an error in the case where the connection is closed.
	///
	/// The protocol name is expected to be checked ahead of calling this method. It is a logic
	/// error to send a notification using an unknown protocol.
	pub async fn reserve_notification(&self) -> Result<Ready<'_>, ()> {
		let mut lock = self.inner.async_channel.lock().await;

		let poll_ready = future::poll_fn(|cx| lock.poll_ready(cx)).await;
		if poll_ready.is_ok() {
			Ok(Ready { lock })
		} else {
			Err(())
		}
	}
}

/// Notification slot is reserved and the notification can actually be sent.
#[must_use]
#[derive(Debug)]
pub struct Ready<'a> {
	/// Guarded channel. The channel inside is guaranteed to not be full.
	lock: FuturesMutexGuard<'a, mpsc::Sender<NotificationsSinkMessage>>,
}

impl<'a> Ready<'a> {
	/// Consumes this slots reservation and actually queues the notification.
	///
	/// Returns an error if the substream has been closed.
	pub fn send(mut self, notification: impl Into<Vec<u8>>) -> Result<(), ()> {
		self.lock
			.start_send(NotificationsSinkMessage::Notification { message: notification.into() })
			.map_err(|_| ())
	}
}

/// Error specific to the collection of protocols.
#[derive(Debug, thiserror::Error)]
pub enum NotifsHandlerError {
	#[error("Channel of synchronous notifications is full.")]
	SyncNotificationsClogged,
}

impl ConnectionHandler for NotifsHandler {
	type InEvent = NotifsHandlerIn;
	type OutEvent = NotifsHandlerOut;
	type Error = NotifsHandlerError;
	type InboundProtocol = UpgradeCollec<NotificationsIn>;
	type OutboundProtocol = NotificationsOut;
	// Index within the `out_protocols`.
	type OutboundOpenInfo = usize;
	type InboundOpenInfo = ();

	fn listen_protocol(&self) -> SubstreamProtocol<Self::InboundProtocol, ()> {
		let protocols = self
			.protocols
			.iter()
			.map(|p| p.in_upgrade.clone())
			.collect::<UpgradeCollec<_>>();

		SubstreamProtocol::new(protocols, ())
	}

	fn on_connection_event(
		&mut self,
		event: ConnectionEvent<
			'_,
			Self::InboundProtocol,
			Self::OutboundProtocol,
			Self::InboundOpenInfo,
			Self::OutboundOpenInfo,
		>,
	) {
		match event {
			ConnectionEvent::FullyNegotiatedInbound(inbound) => {
				let (mut in_substream_open, protocol_index) = inbound.protocol;
				let protocol_info = &mut self.protocols[protocol_index];

				match protocol_info.state {
					State::Closed { pending_opening } => {
						self.events_queue.push_back(ConnectionHandlerEvent::Custom(
							NotifsHandlerOut::OpenDesiredByRemote { protocol_index },
						));

						protocol_info.state = State::OpenDesiredByRemote {
							in_substream: in_substream_open.substream,
							pending_opening,
						};
					},
					State::OpenDesiredByRemote { .. } => {
						// If a substream already exists, silently drop the new one.
						// Note that we drop the substream, which will send an equivalent to a
						// TCP "RST" to the remote and force-close the substream. It might
						// seem like an unclean way to get rid of a substream. However, keep
						// in mind that it is invalid for the remote to open multiple such
						// substreams, and therefore sending a "RST" is the most correct thing
						// to do.
						return
					},
					State::Opening { ref mut in_substream, .. } |
					State::Open { ref mut in_substream, .. } => {
						if in_substream.is_some() {
							// Same remark as above.
							return
						}

						// Create `handshake_message` on a separate line to be sure that the
						// lock is released as soon as possible.
						let handshake_message = protocol_info.config.handshake.read().clone();
						in_substream_open.substream.send_handshake(handshake_message);
						*in_substream = Some(in_substream_open.substream);
					},
				}
			},
			ConnectionEvent::FullyNegotiatedOutbound(outbound) => {
				let (new_open, protocol_index) = (outbound.protocol, outbound.info);

				match self.protocols[protocol_index].state {
					State::Closed { ref mut pending_opening } |
					State::OpenDesiredByRemote { ref mut pending_opening, .. } => {
						debug_assert!(*pending_opening);
						*pending_opening = false;
					},
					State::Open { .. } => {
						error!(target: "sub-libp2p", "☎️ State mismatch in notifications handler");
						debug_assert!(false);
					},
					State::Opening { ref mut in_substream, inbound } => {
						let (async_tx, async_rx) = mpsc::channel(ASYNC_NOTIFICATIONS_BUFFER_SIZE);
						let (sync_tx, sync_rx) = mpsc::channel(SYNC_NOTIFICATIONS_BUFFER_SIZE);
						let notifications_sink = NotificationsSink {
							inner: Arc::new(NotificationsSinkInner {
								peer_id: self.peer_id,
								async_channel: FuturesMutex::new(async_tx),
								sync_channel: Mutex::new(Some(sync_tx)),
							}),
						};

						self.protocols[protocol_index].state = State::Open {
							notifications_sink_rx: stream::select(async_rx.fuse(), sync_rx.fuse())
								.peekable(),
							out_substream: Some(new_open.substream),
							in_substream: in_substream.take(),
						};

						self.events_queue.push_back(ConnectionHandlerEvent::Custom(
							NotifsHandlerOut::OpenResultOk {
								protocol_index,
								negotiated_fallback: new_open.negotiated_fallback,
								endpoint: self.endpoint.clone(),
								received_handshake: new_open.handshake,
								notifications_sink,
								inbound,
							},
						));
					},
				}
			},
			ConnectionEvent::AddressChange(_address_change) => {},
			ConnectionEvent::DialUpgradeError(dial_upgrade_error) => match self.protocols
				[dial_upgrade_error.info]
				.state
			{
				State::Closed { ref mut pending_opening } |
				State::OpenDesiredByRemote { ref mut pending_opening, .. } => {
					debug_assert!(*pending_opening);
					*pending_opening = false;
				},

				State::Opening { .. } => {
					self.protocols[dial_upgrade_error.info].state =
						State::Closed { pending_opening: false };

					self.events_queue.push_back(ConnectionHandlerEvent::Custom(
						NotifsHandlerOut::OpenResultErr { protocol_index: dial_upgrade_error.info },
					));
				},

				// No substream is being open when already `Open`.
				State::Open { .. } => debug_assert!(false),
			},
			ConnectionEvent::ListenUpgradeError(_listen_upgrade_error) => {},
		}
	}

	fn on_behaviour_event(&mut self, message: NotifsHandlerIn) {
		match message {
			NotifsHandlerIn::Open { protocol_index } => {
				let protocol_info = &mut self.protocols[protocol_index];
				match &mut protocol_info.state {
					State::Closed { pending_opening } => {
						if !*pending_opening {
							let proto = NotificationsOut::new(
								protocol_info.config.name.clone(),
								protocol_info.config.fallback_names.clone(),
								protocol_info.config.handshake.read().clone(),
								protocol_info.config.max_notification_size,
							);

							self.events_queue.push_back(
								ConnectionHandlerEvent::OutboundSubstreamRequest {
									protocol: SubstreamProtocol::new(proto, protocol_index)
										.with_timeout(OPEN_TIMEOUT),
								},
							);
						}

						protocol_info.state = State::Opening { in_substream: None, inbound: false };
					},
					State::OpenDesiredByRemote { pending_opening, in_substream } => {
						let handshake_message = protocol_info.config.handshake.read().clone();

						if !*pending_opening {
							let proto = NotificationsOut::new(
								protocol_info.config.name.clone(),
								protocol_info.config.fallback_names.clone(),
								handshake_message.clone(),
								protocol_info.config.max_notification_size,
							);

							self.events_queue.push_back(
								ConnectionHandlerEvent::OutboundSubstreamRequest {
									protocol: SubstreamProtocol::new(proto, protocol_index)
										.with_timeout(OPEN_TIMEOUT),
								},
							);
						}

						in_substream.send_handshake(handshake_message);

						// The state change is done in two steps because of borrowing issues.
						let in_substream = match mem::replace(
							&mut protocol_info.state,
							State::Opening { in_substream: None, inbound: false },
						) {
							State::OpenDesiredByRemote { in_substream, .. } => in_substream,
							_ => unreachable!(),
						};
						protocol_info.state =
							State::Opening { in_substream: Some(in_substream), inbound: true };
					},
					State::Opening { .. } | State::Open { .. } => {
						// As documented, it is forbidden to send an `Open` while there is already
						// one in the fly.
						error!(target: "sub-libp2p", "opening already-opened handler");
						debug_assert!(false);
					},
				}
			},

			NotifsHandlerIn::Close { protocol_index } => {
				match self.protocols[protocol_index].state {
					State::Open { .. } => {
						self.protocols[protocol_index].state =
							State::Closed { pending_opening: false };
					},
					State::Opening { .. } => {
						self.protocols[protocol_index].state =
							State::Closed { pending_opening: true };

						self.events_queue.push_back(ConnectionHandlerEvent::Custom(
							NotifsHandlerOut::OpenResultErr { protocol_index },
						));
					},
					State::OpenDesiredByRemote { pending_opening, .. } => {
						self.protocols[protocol_index].state = State::Closed { pending_opening };
					},
					State::Closed { .. } => {},
				}

				self.events_queue.push_back(ConnectionHandlerEvent::Custom(
					NotifsHandlerOut::CloseResult { protocol_index },
				));
			},
		}
	}

	fn connection_keep_alive(&self) -> KeepAlive {
		// `Yes` if any protocol has some activity.
		if self.protocols.iter().any(|p| !matches!(p.state, State::Closed { .. })) {
			return KeepAlive::Yes
		}

		// A grace period of `INITIAL_KEEPALIVE_TIME` must be given to leave time for the remote
		// to express desire to open substreams.
		KeepAlive::Until(self.when_connection_open + INITIAL_KEEPALIVE_TIME)
	}

	fn poll(
		&mut self,
		cx: &mut Context,
	) -> Poll<
		ConnectionHandlerEvent<
			Self::OutboundProtocol,
			Self::OutboundOpenInfo,
			Self::OutEvent,
			Self::Error,
		>,
	> {
		if let Some(ev) = self.events_queue.pop_front() {
			return Poll::Ready(ev)
		}

		// For each open substream, try send messages from `notifications_sink_rx` to the
		// substream.
		for protocol_index in 0..self.protocols.len() {
			if let State::Open {
				notifications_sink_rx, out_substream: Some(out_substream), ..
			} = &mut self.protocols[protocol_index].state
			{
				loop {
					// Only proceed with `out_substream.poll_ready_unpin` if there is an element
					// available in `notifications_sink_rx`. This avoids waking up the task when
					// a substream is ready to send if there isn't actually something to send.
					match Pin::new(&mut *notifications_sink_rx).as_mut().poll_peek(cx) {
						Poll::Ready(Some(&NotificationsSinkMessage::ForceClose)) =>
							return Poll::Ready(ConnectionHandlerEvent::Close(
								NotifsHandlerError::SyncNotificationsClogged,
							)),
						Poll::Ready(Some(&NotificationsSinkMessage::Notification { .. })) => {},
						Poll::Ready(None) | Poll::Pending => break,
					}

					// Before we extract the element from `notifications_sink_rx`, check that the
					// substream is ready to accept a message.
					match out_substream.poll_ready_unpin(cx) {
						Poll::Ready(_) => {},
						Poll::Pending => break,
					}

					// Now that the substream is ready for a message, grab what to send.
					let message = match notifications_sink_rx.poll_next_unpin(cx) {
						Poll::Ready(Some(NotificationsSinkMessage::Notification { message })) =>
							message,
						Poll::Ready(Some(NotificationsSinkMessage::ForceClose)) |
						Poll::Ready(None) |
						Poll::Pending => {
							// Should never be reached, as per `poll_peek` above.
							debug_assert!(false);
							break
						},
					};

					let _ = out_substream.start_send_unpin(message);
					// Note that flushing is performed later down this function.
				}
			}
		}

		// Flush all outbound substreams.
		// When `poll` returns `Poll::Ready`, the libp2p `Swarm` may decide to no longer call
		// `poll` again before it is ready to accept more events.
		// In order to make sure that substreams are flushed as soon as possible, the flush is
		// performed before the code paths that can produce `Ready` (with some rare exceptions).
		// Importantly, however, the flush is performed *after* notifications are queued with
		// `Sink::start_send`.
		for protocol_index in 0..self.protocols.len() {
			match &mut self.protocols[protocol_index].state {
				State::Open { out_substream: out_substream @ Some(_), .. } => {
					match Sink::poll_flush(Pin::new(out_substream.as_mut().unwrap()), cx) {
						Poll::Pending | Poll::Ready(Ok(())) => {},
						Poll::Ready(Err(_)) => {
							*out_substream = None;
							let event = NotifsHandlerOut::CloseDesired { protocol_index };
							return Poll::Ready(ConnectionHandlerEvent::Custom(event))
						},
					};
				},

				State::Closed { .. } |
				State::Opening { .. } |
				State::Open { out_substream: None, .. } |
				State::OpenDesiredByRemote { .. } => {},
			}
		}

		// Poll inbound substreams.
		for protocol_index in 0..self.protocols.len() {
			// Inbound substreams being closed is always tolerated, except for the
			// `OpenDesiredByRemote` state which might need to be switched back to `Closed`.
			match &mut self.protocols[protocol_index].state {
				State::Closed { .. } |
				State::Open { in_substream: None, .. } |
				State::Opening { in_substream: None, .. } => {},

				State::Open { in_substream: in_substream @ Some(_), .. } =>
					match Stream::poll_next(Pin::new(in_substream.as_mut().unwrap()), cx) {
						Poll::Pending => {},
						Poll::Ready(Some(Ok(message))) => {
							let event = NotifsHandlerOut::Notification { protocol_index, message };
							return Poll::Ready(ConnectionHandlerEvent::Custom(event))
						},
						Poll::Ready(None) | Poll::Ready(Some(Err(_))) => *in_substream = None,
					},

				State::OpenDesiredByRemote { in_substream, pending_opening } =>
					match NotificationsInSubstream::poll_process(Pin::new(in_substream), cx) {
						Poll::Pending => {},
						Poll::Ready(Ok(void)) => match void {},
						Poll::Ready(Err(_)) => {
							self.protocols[protocol_index].state =
								State::Closed { pending_opening: *pending_opening };
							return Poll::Ready(ConnectionHandlerEvent::Custom(
								NotifsHandlerOut::CloseDesired { protocol_index },
							))
						},
					},

				State::Opening { in_substream: in_substream @ Some(_), .. } =>
					match NotificationsInSubstream::poll_process(
						Pin::new(in_substream.as_mut().unwrap()),
						cx,
					) {
						Poll::Pending => {},
						Poll::Ready(Ok(void)) => match void {},
						Poll::Ready(Err(_)) => *in_substream = None,
					},
			}
		}

		// This is the only place in this method that can return `Pending`.
		// By putting it at the very bottom, we are guaranteed that everything has been properly
		// polled.
		Poll::Pending
	}
}

#[cfg(test)]
pub mod tests {
	use super::*;
	use crate::protocol::notifications::upgrade::{
		NotificationsInOpen, NotificationsInSubstreamHandshake, NotificationsOutOpen,
	};
	use asynchronous_codec::Framed;
	use libp2p::{
		core::muxing::SubstreamBox,
		swarm::{handler, ConnectionHandlerUpgrErr},
		Multiaddr,
	};
	use multistream_select::{dialer_select_proto, listener_select_proto, Negotiated, Version};
	use std::{
		collections::HashMap,
		io::{Error, IoSlice, IoSliceMut},
	};
	use tokio::sync::mpsc;
	use unsigned_varint::codec::UviBytes;

	struct OpenSubstream {
		notifications: stream::Peekable<
			stream::Select<
				stream::Fuse<futures::channel::mpsc::Receiver<NotificationsSinkMessage>>,
				stream::Fuse<futures::channel::mpsc::Receiver<NotificationsSinkMessage>>,
			>,
		>,
		_in_substream: MockSubstream,
		_out_substream: MockSubstream,
	}

	pub struct ConnectionYielder {
		connections: HashMap<(PeerId, usize), OpenSubstream>,
	}

	impl ConnectionYielder {
		/// Create new [`ConnectionYielder`].
		pub fn new() -> Self {
			Self { connections: HashMap::new() }
		}

		/// Open a new substream for peer.
		pub fn open_substream(
			&mut self,
			peer: PeerId,
			protocol_index: usize,
			endpoint: ConnectedPoint,
			received_handshake: Vec<u8>,
		) -> NotifsHandlerOut {
			let (async_tx, async_rx) =
				futures::channel::mpsc::channel(ASYNC_NOTIFICATIONS_BUFFER_SIZE);
			let (sync_tx, sync_rx) =
				futures::channel::mpsc::channel(SYNC_NOTIFICATIONS_BUFFER_SIZE);
			let notifications_sink = NotificationsSink {
				inner: Arc::new(NotificationsSinkInner {
					peer_id: peer,
					async_channel: FuturesMutex::new(async_tx),
					sync_channel: Mutex::new(Some(sync_tx)),
				}),
			};
			let (in_substream, out_substream) = MockSubstream::new();

			self.connections.insert(
				(peer, protocol_index),
				OpenSubstream {
					notifications: stream::select(async_rx.fuse(), sync_rx.fuse()).peekable(),
					_in_substream: in_substream,
					_out_substream: out_substream,
				},
			);

			NotifsHandlerOut::OpenResultOk {
				protocol_index,
				negotiated_fallback: None,
				endpoint,
				received_handshake,
				notifications_sink,
				inbound: false,
			}
		}

		/// Attempt to get next pending event from one of the notification sinks.
		pub async fn get_next_event(&mut self, peer: PeerId, set: usize) -> Option<Vec<u8>> {
			let substream = if let Some(info) = self.connections.get_mut(&(peer, set)) {
				info
			} else {
				return None
			};

			futures::future::poll_fn(|cx| match substream.notifications.poll_next_unpin(cx) {
				Poll::Ready(Some(NotificationsSinkMessage::Notification { message })) =>
					Poll::Ready(Some(message)),
				Poll::Pending => Poll::Ready(None),
				Poll::Ready(Some(NotificationsSinkMessage::ForceClose)) | Poll::Ready(None) => {
					panic!("sink closed")
				},
			})
			.await
		}
	}
	struct MockSubstream {
		pub rx: mpsc::Receiver<Vec<u8>>,
		pub tx: mpsc::Sender<Vec<u8>>,
		rx_buffer: BytesMut,
	}

	impl MockSubstream {
		/// Create new substream pair.
		pub fn new() -> (Self, Self) {
			let (tx1, rx1) = mpsc::channel(32);
			let (tx2, rx2) = mpsc::channel(32);

			(
				Self { rx: rx1, tx: tx2, rx_buffer: BytesMut::with_capacity(512) },
				Self { rx: rx2, tx: tx1, rx_buffer: BytesMut::with_capacity(512) },
			)
		}

		/// Create new negotiated substream pair.
		pub async fn negotiated() -> (Negotiated<SubstreamBox>, Negotiated<SubstreamBox>) {
			let (socket1, socket2) = Self::new();
			let socket1 = SubstreamBox::new(socket1);
			let socket2 = SubstreamBox::new(socket2);

			let protos = vec![b"/echo/1.0.0", b"/echo/2.5.0"];
			let (res1, res2) = tokio::join!(
				dialer_select_proto(socket1, protos.clone(), Version::V1),
				listener_select_proto(socket2, protos),
			);

			(res1.unwrap().1, res2.unwrap().1)
		}
	}

	impl AsyncWrite for MockSubstream {
		fn poll_write<'a>(
			self: Pin<&mut Self>,
			_cx: &mut Context<'a>,
			buf: &[u8],
		) -> Poll<Result<usize, Error>> {
			match self.tx.try_send(buf.to_vec()) {
				Ok(_) => Poll::Ready(Ok(buf.len())),
				Err(_) => Poll::Ready(Err(std::io::ErrorKind::UnexpectedEof.into())),
			}
		}

		fn poll_flush<'a>(self: Pin<&mut Self>, _cx: &mut Context<'a>) -> Poll<Result<(), Error>> {
			Poll::Ready(Ok(()))
		}

		fn poll_close<'a>(self: Pin<&mut Self>, _cx: &mut Context<'a>) -> Poll<Result<(), Error>> {
			Poll::Ready(Ok(()))
		}

		fn poll_write_vectored<'a, 'b>(
			self: Pin<&mut Self>,
			_cx: &mut Context<'a>,
			_bufs: &[IoSlice<'b>],
		) -> Poll<Result<usize, Error>> {
			unimplemented!();
		}
	}

	impl AsyncRead for MockSubstream {
		fn poll_read<'a>(
			mut self: Pin<&mut Self>,
			cx: &mut Context<'a>,
			buf: &mut [u8],
		) -> Poll<Result<usize, Error>> {
			match self.rx.poll_recv(cx) {
				Poll::Ready(Some(data)) => self.rx_buffer.extend_from_slice(&data),
				Poll::Ready(None) =>
					return Poll::Ready(Err(std::io::ErrorKind::UnexpectedEof.into())),
				_ => {},
			}

			let nsize = std::cmp::min(self.rx_buffer.len(), buf.len());
			let data = self.rx_buffer.split_to(nsize);
			buf[..nsize].copy_from_slice(&data[..]);

			if nsize > 0 {
				return Poll::Ready(Ok(nsize))
			}

			Poll::Pending
		}

		fn poll_read_vectored<'a, 'b>(
			self: Pin<&mut Self>,
			_cx: &mut Context<'a>,
			_bufs: &mut [IoSliceMut<'b>],
		) -> Poll<Result<usize, Error>> {
			unimplemented!();
		}
	}

	/// Create new [`NotifsHandler`].
	fn notifs_handler() -> NotifsHandler {
		let proto = Protocol {
			config: ProtocolConfig {
				name: "/foo".into(),
				fallback_names: vec![],
				handshake: Arc::new(RwLock::new(b"hello, world".to_vec())),
				max_notification_size: u64::MAX,
			},
			in_upgrade: NotificationsIn::new("/foo", Vec::new(), u64::MAX),
			state: State::Closed { pending_opening: false },
		};

		NotifsHandler {
			protocols: vec![proto],
			when_connection_open: Instant::now(),
			endpoint: ConnectedPoint::Listener {
				local_addr: Multiaddr::empty(),
				send_back_addr: Multiaddr::empty(),
			},
			peer_id: PeerId::random(),
			events_queue: VecDeque::new(),
		}
	}

	// verify that if another substream is attempted to be opened by remote while an inbound
	// substream already exists, the new inbound stream is rejected and closed by the local node.
	#[tokio::test]
	async fn second_open_desired_by_remote_rejected() {
		let mut handler = notifs_handler();
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};

		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the substream is in (partly) opened state
		assert!(std::matches!(handler.protocols[0].state, State::OpenDesiredByRemote { .. }));
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);
			assert!(std::matches!(Pin::new(&mut io2).poll_read(cx, &mut buf), Poll::Pending));
			Poll::Ready(())
		})
		.await;

		// attempt to open another inbound substream and verify that it is rejected
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};

		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the new substream is rejected and closed
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);

			if let Poll::Ready(Err(err)) = Pin::new(&mut io2).poll_read(cx, &mut buf) {
				assert_eq!(err.kind(), std::io::ErrorKind::UnexpectedEof,);
			}

			Poll::Ready(())
		})
		.await;
	}

	#[tokio::test]
	async fn open_rejected_if_substream_is_opening() {
		let mut handler = notifs_handler();
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};

		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the substream is in (partly) opened state
		assert!(std::matches!(handler.protocols[0].state, State::OpenDesiredByRemote { .. }));
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);
			assert!(std::matches!(Pin::new(&mut io2).poll_read(cx, &mut buf), Poll::Pending));
			Poll::Ready(())
		})
		.await;

		// move the handler state to 'Opening'
		handler.on_behaviour_event(NotifsHandlerIn::Open { protocol_index: 0 });
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Opening { in_substream: Some(_), .. }
		));

		// remote now tries to open another substream, verify that it is rejected and closed
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};

		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the new substream is rejected and closed but that the first substream is
		// still in correct state
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);

			if let Poll::Ready(Err(err)) = Pin::new(&mut io2).poll_read(cx, &mut buf) {
				assert_eq!(err.kind(), std::io::ErrorKind::UnexpectedEof,);
			} else {
				panic!("unexpected result");
			}

			Poll::Ready(())
		})
		.await;
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Opening { in_substream: Some(_), .. }
		));
	}

	#[tokio::test]
	async fn open_rejected_if_substream_already_open() {
		let mut handler = notifs_handler();
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the substream is in (partly) opened state
		assert!(std::matches!(handler.protocols[0].state, State::OpenDesiredByRemote { .. }));
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);
			assert!(std::matches!(Pin::new(&mut io2).poll_read(cx, &mut buf), Poll::Pending));
			Poll::Ready(())
		})
		.await;

		// move the handler state to 'Opening'
		handler.on_behaviour_event(NotifsHandlerIn::Open { protocol_index: 0 });
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Opening { in_substream: Some(_), .. }
		));

		// accept the substream and move its state to `Open`
		let (io, _io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_out = NotificationsOutOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsOutSubstream::new(Framed::new(io, codec)),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedOutbound(
			handler::FullyNegotiatedOutbound { protocol: notif_out, info: 0 },
		));

		assert!(std::matches!(
			handler.protocols[0].state,
			State::Open { in_substream: Some(_), .. }
		));

		// remote now tries to open another substream, verify that it is rejected and closed
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);
		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the new substream is rejected and closed but that the first substream is
		// still in correct state
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);

			if let Poll::Ready(Err(err)) = Pin::new(&mut io2).poll_read(cx, &mut buf) {
				assert_eq!(err.kind(), std::io::ErrorKind::UnexpectedEof);
			} else {
				panic!("unexpected result");
			}

			Poll::Ready(())
		})
		.await;
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Open { in_substream: Some(_), .. }
		));
	}

	#[tokio::test]
	async fn fully_negotiated_resets_state_for_closed_substream() {
		let mut handler = notifs_handler();
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the substream is in (partly) opened state
		assert!(std::matches!(handler.protocols[0].state, State::OpenDesiredByRemote { .. }));
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);
			assert!(std::matches!(Pin::new(&mut io2).poll_read(cx, &mut buf), Poll::Pending));
			Poll::Ready(())
		})
		.await;

		// first instruct the handler to open a connection and then close it right after
		// so the handler is in state `Closed { pending_opening: true }`
		handler.on_behaviour_event(NotifsHandlerIn::Open { protocol_index: 0 });
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Opening { in_substream: Some(_), .. }
		));

		handler.on_behaviour_event(NotifsHandlerIn::Close { protocol_index: 0 });
		assert!(std::matches!(handler.protocols[0].state, State::Closed { pending_opening: true }));

		// verify that if the the outbound substream is successfully negotiated, the state is not
		// changed as the substream was commanded to be closed by the handler.
		let (io, _io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_out = NotificationsOutOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsOutSubstream::new(Framed::new(io, codec)),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedOutbound(
			handler::FullyNegotiatedOutbound { protocol: notif_out, info: 0 },
		));

		assert!(std::matches!(
			handler.protocols[0].state,
			State::Closed { pending_opening: false }
		));
	}

	#[tokio::test]
	async fn fully_negotiated_resets_state_for_open_desired_substream() {
		let mut handler = notifs_handler();
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the substream is in (partly) opened state
		assert!(std::matches!(handler.protocols[0].state, State::OpenDesiredByRemote { .. }));
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);
			assert!(std::matches!(Pin::new(&mut io2).poll_read(cx, &mut buf), Poll::Pending));
			Poll::Ready(())
		})
		.await;

		// first instruct the handler to open a connection and then close it right after
		// so the handler is in state `Closed { pending_opening: true }`
		handler.on_behaviour_event(NotifsHandlerIn::Open { protocol_index: 0 });
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Opening { in_substream: Some(_), .. }
		));

		handler.on_behaviour_event(NotifsHandlerIn::Close { protocol_index: 0 });
		assert!(std::matches!(handler.protocols[0].state, State::Closed { pending_opening: true }));

		// attempt to open another inbound substream and verify that it is rejected
		let (io, _io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		assert!(std::matches!(
			handler.protocols[0].state,
			State::OpenDesiredByRemote { pending_opening: true, .. }
		));

		// verify that if the the outbound substream is successfully negotiated, the state is not
		// changed as the substream was commanded to be closed by the handler.
		let (io, _io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_out = NotificationsOutOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsOutSubstream::new(Framed::new(io, codec)),
		};

		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedOutbound(
			handler::FullyNegotiatedOutbound { protocol: notif_out, info: 0 },
		));

		assert!(std::matches!(
			handler.protocols[0].state,
			State::OpenDesiredByRemote { pending_opening: false, .. }
		));
	}

	#[tokio::test]
	async fn dial_upgrade_error_resets_closed_outbound_state() {
		let mut handler = notifs_handler();
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the substream is in (partly) opened state
		assert!(std::matches!(handler.protocols[0].state, State::OpenDesiredByRemote { .. }));
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);
			assert!(std::matches!(Pin::new(&mut io2).poll_read(cx, &mut buf), Poll::Pending));
			Poll::Ready(())
		})
		.await;

		// first instruct the handler to open a connection and then close it right after
		// so the handler is in state `Closed { pending_opening: true }`
		handler.on_behaviour_event(NotifsHandlerIn::Open { protocol_index: 0 });
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Opening { in_substream: Some(_), .. }
		));

		handler.on_behaviour_event(NotifsHandlerIn::Close { protocol_index: 0 });
		assert!(std::matches!(handler.protocols[0].state, State::Closed { pending_opening: true }));

		// inject dial failure to an already closed substream and verify outbound state is reset
		handler.on_connection_event(handler::ConnectionEvent::DialUpgradeError(
			handler::DialUpgradeError { info: 0, error: ConnectionHandlerUpgrErr::Timeout },
		));
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Closed { pending_opening: false }
		));
	}

	#[tokio::test]
	async fn dial_upgrade_error_resets_open_desired_state() {
		let mut handler = notifs_handler();
		let (io, mut io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		// verify that the substream is in (partly) opened state
		assert!(std::matches!(handler.protocols[0].state, State::OpenDesiredByRemote { .. }));
		futures::future::poll_fn(|cx| {
			let mut buf = Vec::with_capacity(512);
			assert!(std::matches!(Pin::new(&mut io2).poll_read(cx, &mut buf), Poll::Pending));
			Poll::Ready(())
		})
		.await;

		// first instruct the handler to open a connection and then close it right after
		// so the handler is in state `Closed { pending_opening: true }`
		handler.on_behaviour_event(NotifsHandlerIn::Open { protocol_index: 0 });
		assert!(std::matches!(
			handler.protocols[0].state,
			State::Opening { in_substream: Some(_), .. }
		));

		handler.on_behaviour_event(NotifsHandlerIn::Close { protocol_index: 0 });
		assert!(std::matches!(handler.protocols[0].state, State::Closed { pending_opening: true }));

		let (io, _io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::NotSent,
			),
		};
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));

		assert!(std::matches!(
			handler.protocols[0].state,
			State::OpenDesiredByRemote { pending_opening: true, .. }
		));

		// inject dial failure to an already closed substream and verify outbound state is reset
		handler.on_connection_event(handler::ConnectionEvent::DialUpgradeError(
			handler::DialUpgradeError { info: 0, error: ConnectionHandlerUpgrErr::Timeout },
		));
		assert!(std::matches!(
			handler.protocols[0].state,
			State::OpenDesiredByRemote { pending_opening: false, .. }
		));
	}

	#[tokio::test]
	async fn sync_notifications_clogged() {
		let mut handler = notifs_handler();
		let (io, _) = MockSubstream::negotiated().await;
		let codec = UviBytes::default();

		let (async_tx, async_rx) = futures::channel::mpsc::channel(ASYNC_NOTIFICATIONS_BUFFER_SIZE);
		let (sync_tx, sync_rx) = futures::channel::mpsc::channel(1);
		let notifications_sink = NotificationsSink {
			inner: Arc::new(NotificationsSinkInner {
				peer_id: PeerId::random(),
				async_channel: FuturesMutex::new(async_tx),
				sync_channel: Mutex::new(Some(sync_tx)),
			}),
		};

		handler.protocols[0].state = State::Open {
			notifications_sink_rx: stream::select(async_rx.fuse(), sync_rx.fuse()).peekable(),
			out_substream: Some(NotificationsOutSubstream::new(Framed::new(io, codec))),
			in_substream: None,
		};

		notifications_sink.send_sync_notification(vec![1, 3, 3, 7]);
		notifications_sink.send_sync_notification(vec![1, 3, 3, 8]);
		notifications_sink.send_sync_notification(vec![1, 3, 3, 9]);
		notifications_sink.send_sync_notification(vec![1, 3, 4, 0]);

		futures::future::poll_fn(|cx| {
			assert!(std::matches!(
				handler.poll(cx),
				Poll::Ready(ConnectionHandlerEvent::Close(
					NotifsHandlerError::SyncNotificationsClogged,
				))
			));
			Poll::Ready(())
		})
		.await;
	}

	#[tokio::test]
	async fn close_desired_by_remote() {
		let mut handler = notifs_handler();
		let (io, io2) = MockSubstream::negotiated().await;
		let mut codec = UviBytes::default();
		codec.set_max_len(usize::MAX);

		let notif_in = NotificationsInOpen {
			handshake: b"hello, world".to_vec(),
			negotiated_fallback: None,
			substream: NotificationsInSubstream::new(
				Framed::new(io, codec),
				NotificationsInSubstreamHandshake::PendingSend(vec![1, 2, 3, 4]),
			),
		};

		// add new inbound substream but close it immediately and verify that correct events are
		// emitted
		handler.on_connection_event(handler::ConnectionEvent::FullyNegotiatedInbound(
			handler::FullyNegotiatedInbound { protocol: (notif_in, 0), info: () },
		));
		drop(io2);

		futures::future::poll_fn(|cx| {
			assert!(std::matches!(
				handler.poll(cx),
				Poll::Ready(ConnectionHandlerEvent::Custom(
					NotifsHandlerOut::OpenDesiredByRemote { protocol_index: 0 },
				))
			));
			assert!(std::matches!(
				handler.poll(cx),
				Poll::Ready(ConnectionHandlerEvent::Custom(NotifsHandlerOut::CloseDesired {
					protocol_index: 0
				},))
			));
			Poll::Ready(())
		})
		.await;
	}
}